1. Field of the Invention
The present invention relates generally to electrical switchgear apparatus, and more particularly, to an automatic secondary disconnect mechanism for a removable circuit device.
2. Description of the Prior Art
Circuit breakers for medium voltage switchgear applications, or circuit devices in general, are typically housed in a metal enclosure and are removable. So-called "drawout" apparatus are used to move the circuit breakers between a disconnect position, in which the primary contacts of the circuit breaker are fully disengaged from the mating primary contacts within the enclosure, and a connect position in which the primary contacts of the circuit breaker and enclosure are fully engaged. A racking mechanism is usually employed to ensure steady movement and proper alignment of the circuit breaker as it moves between the disconnect and connect positions.
Circuit breakers are typically equipped with a variety of auxiliary electronic devices and related control circuitry, including switches, motors, solenoids and the like, for providing electronic control of the circuit breaker. For example, automatic operation of the circuit breaker's interrupter contacts may be achieved through the use of such auxiliary devices and control circuitry. Of course, power must be supplied to the auxiliary devices and control circuitry at some point.
Typically, power is provided to the auxiliary devices and control circuitry through mating secondary contacts mounted with the circuit breaker and enclosure. At some point during movement of the circuit breaker from the disconnect position to the connect position, the respective secondary contacts on the circuit breaker and enclosure must be engaged so that power is provided to the auxiliary devices and control electronics. Upon removal of the circuit breaker, the secondary contacts must be disengaged. Additionally, it is common to provide a test position between the connect and disconnect positions of the circuit breaker wherein the secondary contacts are engaged, but the primary contacts are not engaged. With the secondary contacts engaged, but the primary contacts disengaged, the auxiliary functions of the circuit breaker can be thoroughly tested in safety while the breaker is "dead". For example, automatic opening and closing of the circuit breaker's interrupter switches can be tested.
It is generally desirable for the secondary contacts to engage as the circuit breaker reaches the test position and to remain engaged as the circuit breaker continues past the test position to the full connect position. Likewise, upon removal of the circuit breaker it is desirable for the secondary contacts to remain engaged as the circuit breaker moves from the connect position back to the test position, but then to disengage as the circuit breaker moves past the test position to the full disconnect position. Mechanisms for connecting and disconnecting the secondary contacts are generally referred to in the art as "secondary disconnects". Although some prior art switchgear apparatus provide a manual secondary disconnect mechanism, it is generally more desirable to provide an automatic secondary disconnect mechanism in which engagement and disengagement of the secondary contacts is incident to the racking movement of the circuit breaker into and out of its enclosure.
Netzel and Ericson et al., U.S. Pat. Nos. 3,188,415 and 4,020,301, respectively, have both tried to meet the aforementioned objectives through the use of a secondary disconnect mechanism wherein the secondary contacts comprise a pair of conductive strips fixedly mounted to the circuit breaker and enclosure in parallel relation. As the circuit breaker moves into the test position, the respective contact strips begin to slide upon each other. As the circuit breaker moves past the test position into the full connect position, the respective contact strips continue to slide against each other, thereby maintaining contact throughout the entire travel of the circuit breaker. When the circuit breaker moves out of the enclosure, the sliding contacts slide off each other. Unfortunately, sliding contacts are subject to increased wear and require more frequent maintenance. Additionally, because proper connection depends upon continuous engagement of the two contact as they slide over each other, sliding contacts are more likely to fail or falter at some point during movement of the circuit breaker. For these reasons, sliding contacts do not provide an acceptable solution.
Bould and Wolfe et al., U.S. Pat. Nos. 4,565,908 and 4,139,748, both describe secondary disconnect mechanisms wherein one of the secondary contact halves or members is fixedly attached to the circuit breaker, while the mating secondary contact member is slidably mounted to a guide pin attached to the switchgear enclosure. Each mechanism uses a spring to bias the movable contact member against the fixed contact member on the circuit breaker. As the circuit breaker moves into the test position, the spring provides sufficient biasing in the opposite direction to allow the mating contact members to engage. Once engaged, the contact members move together along the guide pin against the biasing of the spring as the circuit breaker continues to the full connect position. When the circuit breaker is withdrawn from the enclosure, the force provided by the spring keeps the connector halves together as the circuit breaker moves back out to the test position. As the circuit breaker continuous past the test position toward the disconnect position, the movable contact member reaches the end of its travel along the guide pin and therefore the two contact members disengage. While use of a spring biased, movable contact member may overcome some of the disadvantages of the sliding contact mechanisms described by Netzel and Ericson et al., the present inventors have found that biasing the movable contact member with a spring is undesirable. Most notably, use of a spring to bias the movable secondary contact member makes it difficult to ensure proper alignment of the secondary contact members upon initial engagement at the test position. Additionally, the spring may impede the racking motion of the circuit breaker.
Other secondary disconnect mechanisms are described in U.S. Pat. Nos. 4,743,715 (Gerbert-Gaillard et al), 4,761,521 (Beck et al.), 4,236,189 (Yosida) and 5,043,541 (Krafft et al.). However, each of these secondary disconnect mechanisms is either too complex to implement efficiently or is not fully automatic. Consequently, there is a need for an automatic secondary disconnect mechanism that is inexpensive, uses few moving parts, and overcomes the aforementioned limitations and deficiencies of known secondary disconnect mechanisms. The present invention satisfies this need.